Initially, one should bear in mind that existing multiple signal reception systems were analog having uniform complexity, and while these are inexpensive performance is not good in difficult environments. Most of these systems are short-range devices of the RKE (Remote Keyless Entry), SES (Smart Entry System) or TPMS (Tire Pressure Monitoring System) type and do not use digital noise reduction modules through self-referencing.
Furthermore, with background art, devices have been known that reduce interference in received communication signals, from the specifications of French Patent No. 2846814, French Patent No. 2846815, French Patent No. 2859336 and French Patent No. 2846825. Consequently, these are composed so as to use an adaptive filter whose adaptiveness is controlled in accordance with detection of a target signal in a received communication signal.
In addition, from the specification of French Patent No. 2899052, a receiver for removing interference signals from a received communication signal is known, as shown in FIG. 1. A receiver 20 includes an RF analog front-end circuit 22 capable of receiving an input signal RF from an antenna 24. The RF front-end circuit 22 can filter a portion of the received signal and offset the received signal with an appropriate intermediate frequency for analog/digital conversion and for digital processing by a processing circuit 26. The processing circuit 26 includes a signal analog/digital converter, a regulator 28, an adaptive filter 30, a detector 32 for detecting reception of a target signal, and a controller 40 for controlling the operating mode of the adaptive filter 30 in response to signals 36 and 38 transmitted from the detector 32 and a demodulator 34, respectively.
The adaptive filter 30 uses a self-reference signal (a received signal accompanying delay by a delay Δ) used as a reference signal for an adaptive Wiener filter. The self-reference signal has a stable phase relationship with the received signal in the case of coherent interference. This phase is not stable for incoherent interference. The object of the adaptive filter 30 is to effectively remove all components of coherent (and stable) signals other than the target signal. Consequently, the adaptive filter 30 can selective use an adaptive mode when the presence of a target signal is completely undetected or a non-adaptive mode, that is to say a simple filtering mode, when the presence of a target signal is detected, and through this the desired signal is not removed and continues to be demodulated by passing through the filter 30. The expression “target signal” can mean a signal whose reception is desired prior to the nature of the target signal being confirmed. The control circuit 40 is composed so as to generate a control signal 41, and can control the operating mode of the filter 30 in accordance with whether or not the signal 36 from the detector 32 is the target signal and/or whether or not the signal 38 from the demodulator 34 is an effective demodulated signal (that is to say, whether or not the demodulated signal behaves appropriately and follows the transition timing determined by the modulation method).
Within the scope of the present invention, a number of defects have been confirmed in the conventional solution methods.
As an example, the performance of the above-described system is illustrated in FIGS. 2A and 2B. In FIG. 2A, the power spectrum densities of the received signal (r) and the filtering signal (s) are shown. The received signal includes three coherent, stable interference signals (jc) (for example, a Desired/Undesired ratio DU=−10 dB). Looking at the filtering signal (s), it can be seen that an adaptive Wiener filter is suited for coherent interference.
In FIG. 2B, different examples of the received signal (r) and the filtering signal (s) are shown. In this case, the received signal (r) includes a white noise (n) type interference signal accompanied by a −5 dB SNR (signal-to-noise ratio). In FIG. 2B, it can be seen that the noise level is constantly the same as the filtering signal level, and in addition the shape of the second tone changes. The adaptive Wiener filter is not suitable for this kind of noise signal with no time correlation. The table shown in FIG. 3 shows the performance observed in this kind of receiver in the case of additional white noise (incoherent noise). By directly using the received signal (r) for demodulation, performance is improved by around 3 dB compared to using the filtering signal (s). Accordingly, reduction of the self-reference noise is not well suited to incoherent noise and it can be seen that distortion of the message occurs prior to demodulation.
Consequently, on the one hand the performance of the above-described various types of systems for remotely locking and unlocking such as the RICE type is not good in the case of incoherent interference signals such as white noise or pulse noise, while on the other hand it has been confirmed that convergence of the adaptive filter is slow in the case of intermittent interference signals (such as PWM, ASK, AM interference and the like).
In reality, various existing self-reference methods are applied to coherent interference and noise such as interference from the environment, wide-area or TETRA, or to noise such as carport door opening systems, but a noise-reducing adaptive filter for this kind of system is not effective on incoherent noise such as white noise or pulse noise, and distortion of the message occurs through this. Furthermore, with the above-described system, a module for detecting the presence of the target signal on the basis of energy directs an update to the coefficient of the adaptive filter. In the case of intermittent interference, this detection module produces erroneous determinations and halts updating of the filter at the next energy rise. Thus, convergence of the adaptive filter requires more time than in the case of continuous interference.